1. Field of the Invention
This invention relates to remediation of contaminated soil and water and, more particularly, this invention relates to treatment of soil contaminated with volatile organic compounds such as gasoline by thermally enhanced soil venting and contaminant recovery. The invention also relates to utilizing the mechanical and thermal energy produced by an internal combustion engine to operate the remediation system.
2. Detailed Background
Public officials are becoming aware of the nature and extent of the problem of contamination of the water supply and soil from storage tanks leaking various chemicals into the soil. EPA estimates in the United States place the volume of liquid hazardous waste stored at 20 billion gallons annually These materials are stored at about 4,000 locations. It is also estimated that there are over 3/4 of a million individual motor fuel tanks and over 300,000 gas stations and trucking companies with gasoline storage tanks. As many as 1/3 of the petroleum tanks may be leaking. Over half are estimated to leak at a rate in an excess of 6 gallons per day. This roughly translates into 1.3 million gallons of fuel daily that is lost into the subsurface.
Soil excavation has been the traditional method for decontaminating a site with absorbed hydrocarbon contamination. It is often difficult to assess the full extent of the contamination. Besides being costly, excavating the soil merely changes the location of the contaminated soil. Hazardous waste disposal sites are becoming fewer and fewer and the expense and the regulation concerned with transporting the hazardous waste from the sites to the storage site makes this an unattractive method of disposal. Current laws make the owner of the waste responsible forever for the stored waste whether it is the contaminated dirt or the spent carbon adsorbent.
Soil ventilation is a cost effective way to decontaminate soil. This is effective in locations where the contamination has not reached groundwater. Currently there are two general methods used for remediation of groundwater before it can be discharged into a reinfiltration gallery, sewer or storm drain. These are carbon filtration or air stripping. Carbon filtration is not desirable on highly contaminated sites as the cost of carbon and its associated handling and disposal costs become prohibitive. With air stripping, the cost of carbon is eliminated leaving only replacement costs of packing as the major maintenance expense However, in areas where emissions are controlled, carbon canisters for air polishing are required. When the soil is highly contaminated with hydrocarbon, the associated cost of carbon again becomes prohibitive.
One proposal for the elimination of gasoline vapors is to burn the recovered vapors. See, for example, U.S. Pat. No. 4,846,134, the disclosure of which is incorporated herein by reference. The level of hydrocarbons recovered in the vapor stream is usually not sufficient to maintain combustion by these vapors alone. Either additional fuel must be added to the vapor to sustain combustion or catalyst must be used to maintain combustion.
The in situ treatment of contaminated soil has been carried out by use of neutralizing chemicals and solvents, as well as nutrients and microorganisms to promote in situ biodegradation of the contaminants. In addition, in situ soil flushing has been carried out by injecting solvents or surfactants into the soil to enhance the contaminant solubility. This technique involves the drilling of an extraction well in the contaminated soil zone, the drilling of reinjection wells upgradient of the contaminant area, and the construction of a waste water treatment system Subsequent to the soil treatment, the groundwater is reinjected upgradient of the extraction well, which then leaches through the contaminated soil. The leachate is then collected, treated and reinjected back into the system, creating a closed loop system.
Yet another in situ treatment of contaminated soil involves a process in which production wells are drilled through the contaminated soil zone to a depth just above the water table. Monitoring wells are drilled around the production wells to monitor pressure gradients. A vacuum is then applied to the production wells. Because of the horizontal pressure gradient created in the soil zone by the vacuum pumps, volatiles in the soil percolate and diffuse through the air spaces between the soil particles to the production well. The vacuum established in the soil continuously draws volatile organic compounds and contaminated air from the soil pores, and draws free air through the soil surface down into the soil. The volatiles removed from the monitoring wells are then processed through a liquid-vapor separator. This procedure applies no heat input and is limited in both the rate of contaminant removal and the types of contaminants which can be vaporized.
In another variation of the foregoing technique, the treatment system includes injection wells for injecting steam, hot air and liquid chemicals into the churned soil. Extraction wells operating in a partial vacuum environment provide a horizontal pressure gradient in the soil. The mixture heats the soil and raises the temperature of the chemicals, eventually causing them to evaporate. The evaporated chemicals are drawn horizontally to the extraction wells and piped to a processing system that cools the chemical vapors for conversion into a liquid. The liquid chemicals are then further processed.
In U.S. Pat. No. 4,670,634 the disclosure of which is incorporated herein by reference, there is disclosed a technique for decontaminating soil by the use of radio frequency energy to heat the soil. Electrodes located over the surface of the decontaminated area radiate rf energy into the soil and heat the soil to the extent that gases and vapors are produced. The rising gases and vapors are collected by a vapor barrier which operates under a slight vacuum. While the system appears to be effective, the energy requirements are substantial and costly, and the depth of the heat penetration into the soil is limited.
While the foregoing techniques are somewhat effective in providing in situ decontamination of the soil, many of the shortcomings attendant with such techniques are that the processes incur high operating expenses, require expensive equipment or chemicals, and as a final result are not overall extremely effective in reducing contaminants to a very low level.
From the foregoing, it can be seen that a need exists for a technique for the improved in situ removal of contaminants from a material. A further need exists for an efficient method and apparatus for the in situ decontamination of soil to a very low level, and which reduces the possibility of releasing or expelling such contaminants into the air. Another need exists for apparatus for controlling the injection of a heated gas into contaminated soil. Yet another need exists for an efficient utilization of energy, in heating the injection gas to raise the temperature of the contaminated soil so that volatile and less volatile contaminants can be vaporized.
The Environmental Protection Agency has sponsored tests of in situ contaminant removal by steam injection and also injection of steam air mixtures. The heat provided by either of these methods enhances the vapor concentration of the volatile compounds and thereby aids in their removal of contaminated soil.
Biodegradation of hydrocarbons is disclosed in U.S. Pat. No. 4,765,902, the disclosure in which is incorporated herein by reference The data indicates for some sites that more than half of the contaminant can be removed by evaporation, but the evaporated material may need processing to prevent release to the air. This un-degraded material can present a processing problem or an air contamination problem.